1. Field of the Invention
The invention relates to an inexpensive process for preparing an equimolar mixture of the diastereomeric sugar alcohols .alpha.-D-glucopyranosido-1,6-mannitol and .alpha.-D-glucopyranosido-1,6-sorbitol from .alpha.-D-glucopyranosido-1,6-fructose by continuous catalytic hydrogenation with hydrogen.
The course of the reaction can be illustrated by the following reaction diagram: ##STR1##
2. Description of the Related Art
In the known processes for preparing .alpha.-D-glucopyranosido-1,6-sorbitol (German Patent Specification 2 217 628) and .alpha.-D-glucopyranosido-1,6-mannitol (German Auslegesschrift 2 520 173), a powdered nickel catalyst is used in each case in the discontinuous suspension process (batch process) as hydrogenation catalyst. Discontinuous processes have the disadvantage that their capacity is very small relative to the reaction volume and, consequently, there is a need for large-volume, expensive reaction equipment and storage tanks. Energy consumption and staff requirements are comparatively high in discontinuous processes. Continuous powdered catalyst processes which employ a plurality of hydrogenation reactors connected in cascade avoid some of these disadvantages. It remains necessary, however, to add the powdered catalyst in a controlled manner, to circulate it by pumping and to filter it off quantitatively from the reaction product. The catalyst sludge pumps are subject to a high mechanical wear. The quantitative removal of the powdered catalyst from the reaction product is expensive. Furthermore the danger of reducing the catalyst activity as a result of the additional operations very rapidly is great. It is therefore advantageous to arrange for the reaction to proceed over a fixed catalyst. Such a catalyst must have a high activity which must not diminish over a prolonged period of time because frequent catalyst replacements in fixed-bed reactions are also expensive.
EP-A 152 779 discloses a process for the continuous hydrogenation of .alpha.-D-glucopyranosido-1,6-fructose to form a mixture of the sugar alcohols .alpha.-D-glucopyranosido-1,6-mannitol and -sorbitol over support-free molded bodies of elements of group VIIIA of the periodic table, in which said support-free molded bodies are preferably produced by compacting and/or bonding metal powders.
DE-A 4 416 115 discloses a process for the continuous hydrogenation of .alpha.-D-glucopyranosido-1,6-fructose to form a mixture of the corresponding sugar alcohols over support-free molded bodies of the iron subgroup of group VIIIA of the periodic table with elements of group VIA. Unfortunately, in these processes, the two sugar alcohols are not produced in equimolar amounts, which would be desirable for physical reasons (always the same freezing point of the mixture) and is necessary for use in pharmaceutical applications (prescribed establishment of the composition), but in different quantitative ratios in each case depending on reaction temperature. It is furthermore desirable to increase the hourly catalyst loading markedly and to reduce the catalyst costs further. In addition, it is always desirable to carry out a process at the lowest possible temperature in order to reduce energy costs.